This invention relates to a cathode ray tube for use as a light source.
Light sources for display purposes have previously been various types of incandescent lamps or monochromatic miniature cathode ray tubes. The incandescent lamps have unsatisfactory brightness, short lifetime and are difficult to maintain. Also the conventional type of monochromatic cathode ray tubes have comprised an electron gun disposed therein to emit an electron beam and a phosphor screen irradiated with the electron beam after the deflection thereof whereby the phosphor screen luminesces. This has resulted in the necessity of using a deflecting system for deflecting the electron beam and a complicated driving circuit. Accordingly, there has been the disadvantage that, with a multitude of such cathode ray tubes arranged in a predetermined pattern, it is very difficult to drive them simultaneously.
A conventional electron gun disposed in such a cathode ray tube has included a cathode electrode, a first cylindrical grid electrode, a second cylindrical grid electrode and a third cylindrical grid electrode, disposed in coaxial spaced relationship and supplied with respective voltages as desired. An electron beam emitted from the cathode electrode passes through a crossover point and then diverges toward the phosphor screen involved until it forms a circular luminescent spot thereon. The luminescent spot has a diameter which increases as the voltage applied to the phosphor screen decreases and vice versa. Even though the luminescent spot can be given the required diameter on the screen by decreasing the voltage, the brightness of the luminescent spot is too dark to permit the resulting cathode ray tube to be employed as a light source.
Also in order to maintain the interior of such a cathode ray tube under a high vacuum, a getter, for example barium has been required to be scattered therein. This has caused the barium to adhere to stem leads connected to the electron gun and to discharge thereacross resulting malfunction and unrequired luminescence which decreases the lifetime of the cathode ray tube used as a light source.
Accordingly it is an object of the present invention to provide a new and improved cathode ray tube for use as a light source having a high brightness characteristic.
It is another object of the present invention to provide a miniature cathode ray tube for use as a light source.
It is still another object of the present invention to provide a new and improved cathode ray tube for use as a light source for forming a luminescent spot having any desired diameter on a phosphor screen involved.
It is a different object of the present invention to prevent a cathode ray tube used as a light source from malfunctioning and luminescing unnecessarily.